See, this is what's bonkers to me. What's gonna be done on the Providence Line north of Canton Junction if you don't electrify the Stoughton Branch right away? Just keep co-mingling diesel push-pulls with EMUs and Amtrak? Won't that kneecap the frequencies north of there? At that point, why bother?
It's not going to kneecap frequencies at all. For one, the Needham Line is a much bigger toilet clog on the SW Corridor than any other branch schedule. Two, the Franklin Line mucks things up much more than Stoughton when it has to cross over to get in position to turn out at Readville; they've already proposed interlining it via Fairmount to get it off the NEC. Three, Track 4 being reinstated from Green St. to Readville will do a lot to loosen things up. Four, Hyde Park--mainly a Providence/Stoughton stop--can or will be closed if Fairmount Station gets a frequency bump, meaning that in tandem with Franklin's removal everything will be single-file from Forest Hills to 128.
Finally, diesels reach exactly the same top and cruising speed as EMU's. Acceleration is poorer, but the difference is minimized when a line has wide stop spacing (like the NEC) and the trains are reasonable length and run often enough that they don't have to be sardine-packed with 6 bi-level cars of weighted-down human flesh. EMU's being able to maintain their acceleration profile under
all levels of load is where you can plot a faster schedule by stripping out some of the excess cushion for variable dwells. That matters for packing the schedule. And there are numerous other things that make them more resource-efficient and flexible for the job. But the only place where a diesel is going to get caught straight-up lagging on the NEC is coming out of the station stop at Ruggles...and, still, Needham > Franklin > Stoughton on that worry list. Once you clear Forest Hills and start on the straightaway the travel time to 128 and Canton is not much different vehicle-to-vehicle.
I think we're severely overestimating the size of the performance difference here. Diesels have all kinds of if's and but's for efficiency vs. service density, but they can most definitely haul ass between stops.
Or is the alternative to run a Princeton-style diesel dinky back and forth between Stoughton and Canton Junction with timed transfers? I'm sure that'll be popular (said sarcastically).
Don't joke...they might actually float that one as a lead trial balloon. For
Needham, not Stoughton, because Needham is where RER frequencies are impossible with the SW Corridor infrastructure.
Besides, splitting electrified services at Canton Junction makes sense. It basically gives you a pre-built infrastructure to support short-turns for Urban Rail, without needing to gum up the environmentally sensitive areas around Route 128/University Park for an extra track. (Which, yeah, eventually will need to come with future Amtrak service, but that's a ways off.)
It's unlikely there's going to be any Urban Rail short-turns that way. The need to re-engage Amtrak dispatch at Readville is going to hiccup a few too many Fairmount frequencies. I know they're evaluating that in the Rail Vision study, but they stand a much better chance doing it at Dedham Corporate instead of Westwood because the unified T dispatch ensures there won't be chance pauses to cushion around. Westwood turns are more an NSRL thing (i.e. why the station has slack space to eventually build out to 5-6 platform tracks).
They need the extra track @ 128 anyway. One of the most facepalming mistakes South Coast FAIL made was not speccing any changes whatsoever to the NEC layout, despite knowing they couldn't get enough trains cleanly out there to do a schedule that was worth a damn and knowing that the NEC FUTURE study would be offering up some grist for expanding track capacity.
128's two platforms can each be turned into 2-track islands for 4 total platform tracks. The ROW through Neponset Reservation is already graded for extending quad-track between the Readville freight yard leads and 128 and already has its catenary supports spaced for quad; it only needs drainage fortifications at the edges of the grading to pass muster with the EIS. Between Dedham St. and Canton Jct. it's 2-track framed by rock embankment, so won't be expandable until Amtrak chucks in some serious $$$ for their further-future needs. But Canton Jct. station itself, currently a bottleneck because of the way the Stoughton platform overspills the junction and the lack of passing options on the NEC side, is silly-easy to fix. Shift the Stoughton inbound platform behind the depot building, and take 1 parking row on the NEC southbound side to shift the platform back and add a third track before the Viaduct so Amtrak can pass a stopped Providence train (there's even remnants of a 3rd maintenance-of-way track on that side in the parking lot).
Have the contiguous 4 tracks from Green St. to past 128, separation of CR and high-speed traffic at 128, and sorting opportunities at 128 and Canton Jct. stations and there should be no traffic bottlenecks affecting an RER schedule being sent to Stoughton/Taunton/beyond. And certainly no reason to worry that one last diesel schedule amidst a sea of electrics is somehow going to send everything to a screeching halt.
Do you see now why you fix the capacity ceiling BEFORE making go/no-go decision on electrifying Stoughton? Target fixation on wires means there's no attention being put to the kind of common-sense things that would actually ensure workable schedules. SCR got broken by the Army Corps single-tracking the swamp...they made the BS electrification requirement to try to cover up that colossal cock-up...and the Phase II house of cards has all its credibility staked to electric-or-bust so everyone forgot about the NEC.
EASY stuff on the NEC. Like...I doubt the Neponset Reservation is even going to be that hard to permit given the grading they have to work with.
Also...keep in mind, if you are so gung-ho to electrify Stoughton now that means you CAN'T electrify Riverside now. Sharon sub does not have enough slack capacity. Which line is going to see more frequent service and move more people? Urban Rail to Riverside, hands-down; Stoughton is out-of-range for the intra-128 services. Further, when you do build SCR Phase II the sub that handles the mainline to Taunton is going to need to get placed somewhere around Route 106 in Easton...where the same set of huge high-voltage lines that supply Sharon sub cross the Stoughton ROW. You do not want to predicate your electrical buildout today on a line terminus in Stoughton without knowing if it's ever going to get extended to Taunton...because you'll have completely wasted money and effort on transmission infrastructure elsewhere that will have to be re-sectioned in the future. Again, see Riverside and throwing away an easy-grab opportunity for denser service.
Stoughton is the wrong place to be getting OCD. Think service levels, service levels, service levels on where most bang-for-buck is going to get apportioned on the initial investment. Emphasis on "initial"; you can always tend to it later with or without SCR. But as far as Sharon sub is concerned we need that very first electric investment to pack the absolute most frequent possible service you can cram onto it. Stoughton ain't it. Not like Riverside is.